Feasibility of Stereotactic MR-Guided Adaptive Radiotherapy in Localized Prostate Cancer

Abstract

Extreme hypofractionation by use of SBRT is being frequently used in localized prostate cancer (Pca), after the early studies have resulted in favorable outcome and acceptable toxicity. Stereotactic MR-guided Adaptive Radiotherapy (SMART) has the advantages for markerless, MR guidance with daily adaptation and smaller PTV margins. We aimed to report our initial experience with SMART in localized Pca. Between 09/18-10/19, 35 consecutive patients with T1c-T3aN0M0 Pca were treated with MRidian MRLINAC (ViewRay, OH). Risk groups were low in 20%, intermediate in 57% and high in 23%. MR and CT simulation was done after 3 hours of fasting, fleet enema and 40 mins after 250 cc water. Isotropic PTV margin was 3 mm over CTV (prostate+/-seminal vesicles). Static IMRT plans to give 3625 cGy/5 fractions were studied. 26% received androgen ablation. All patients were treated every other day. Median follow up time was 7 (Range: 1-14) months. Plan adaptation was done in 74% of 175 fractions. The reasons for plan adaptation were target volume coverage in 38%, OAR violation in 26% and both in 32%. Overall treatment time was median 45 min (29-95 min, range). Acute GU toxicity was seen in 34% as Gr1 and in 11% as Gr2. GI toxicity was seen as Gr 1 in 6%. Alpha blocker was needed during treatment for urinary symptoms in 23% of patients. No grade 3 or higher acute or late toxicity was observed. Early PSA kinetics after treatment were positive as expected from SBRT. Only 1 high risk patient developed systemic metastasis and another patient developed biochemical failure. SMART is a tolerable and convenient markerless treatment method for localized Pca patients. Our early results support low toxicity with high rate of local control.